Antenna and radome development
Antenna concepts have changed significantly over the years. They have gone from the parabolic dish to the flat panel. Parabolic antennas had a tendency to emit side lobes which detract from the overall power output. Most radar antennas have the ability to pitch up and down as well as sweep side to side. The antenna can be automatically stabilized by the inertial system to keep the beam focused on the aircraft path during roll and pitch conditions.
The antenna is protected from the elements by a component called a radome. Certain physical and electrical properties have to be engineered into this covering so they are strong enough to withstand air loads and contoured to minimize drag. Physical properties vary with the shape, size, and performance of the aircraft on which it is installed.
Electrically, it should permit the transmitted signals and return echoes to pass freely. This should be accomplished with minimum distortion and absorption. This is accomplished by construction techniques including materials, thickness, and even shape. A very small variation in physical thickness may cause a sizable variation in electrical thickness. Radar efficiency, definition, and accuracy depend upon a clear, nondistorted, reflection-free antenna view through the radome. Consequently, they should be precisely built for optimum performance.
Top: Radar antenna and RT unit combined. Bottom: Transmit inhibit switch installed on RT unit. Always make sure the transmitter is deactive around
There are two general types of radar covering, the "thin wall" and "sandwich" types. Thin wall radomes are considered to be thin relative to the wavelength of the radar. They are generally useful when the radar frequency is low enough to permit a skin thickness that will satisfy the structural requirements. Sandwich radomes consist of two or more plastic skins separated by a dielectric core. The core may consist of honeycomb plastic sections, hollow flutes, or foam plastic. The dielectric and separation of the skins will depend upon the wavelength of the radar frequency.
Probably the most frequent damage to radomes is holes in the structure caused by static discharges. These can be large holes that are readily apparent or small pinholes that are almost imperceptible. Any hole, regardless of size, can cause major damage. Moisture can enter the wall and cause internal delamination. If freezing occurs, more serious damage results and when enough moisture collects, the radiation pattern will be distorted and the transmitted signals along with the return echoes will be invalid. Other types of damage include dents and scratches, but most types of damage can be found through detailed inspections.
A radome having undergone major repairs should be tested to verify that its electrical properties have not been impaired. Testing requires specialized equipment. Improper repair may cause loss of signal, distortion, and displacement of targets, plus clutter the display to obscure possible storm cells. Some examples of improper repair include the use of inappropriate materials, patches of varying thickness, overlapping repairs, and excessive paint thickness or metal based paint.
U.S. FAA Advisory Circular 43-14 provides a guide for radome inspection and maintenance. However, specific aircraft documentation should always be consulted prior to any repairs.
Well now it's time to get back to my bratwurst, but this time I think I'll put it on the grill.
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